you can try wrap deformer as quick and dirty solution.
more clean, though more memory consuming would be to add low poly rig to high poly rig as an influence (skinCluster supports poly influences now). it gives much better results than wrap deformer. you can also break your low poly into more separate parts and add to high poly as multiple influences.
skin cluster+influences approach, however, requires that even your highrez is somewhere around 30k verts, more than that causes huge memory problems (more than 3gb memory just for skin cluster). if you're getting this, then you need more workarounds - like, break your lowrez AND highrez into smaller parts (say, left leg, right leg, torso, left arm, right arm, head), "skin-wrap" them separately, then combine highrez version back. this results in very heavy construct, but allows you to reduce memory consumption to something like four times (5x10k highrez meshes take about 600mb ram, down from 3-4gb if it was one 50k mesh).